The present invention relates to a fuel cell and, more particularly, to a solid oxide fuel cell.
A fuel cell is a power generating device which is provided with an anode (fuel electrode) on one side and a cathode (air electrode) on the other side with an electrolyte being held therebetween, and generates power by allowing a fuel gas supplied to the anode side to react electrochemically with an oxidizer gas supplied to the cathode side via the electrolyte. A solid oxide fuel cell, which is one kind of fuel cells, not only has high power generating efficiency but also is operated at a high temperature of 600 to 1000° C., so that it can carry out a fuel reforming reaction in the cell without the use of a noble metal catalyst, and can use a variety of fuels. Also, since the cell system construction is simple, the solid oxide fuel cell has a potential for reducing the cost as compared with other types of fuel cells. Naturally, the exhaust heat is easily used because of its high temperature, and hence the solid oxide fuel cell has a characteristic of being easily used to form not only a cogeneration system but also a hybrid system with a gas turbine and other equipment.
The high cell working temperature of 600 to 1000° C. offers the above-described advantages, but on the other hand, has a disadvantage that it is very difficult to maintain the temperature in the cell uniformly. If the variations in temperature distribution in a cell reaction region are large, the electrical resistance of a low-temperature portion becomes higher than that of a high-temperature portion, and hence the cell current is turned down. On the other hand, in the high-temperature portion, the electrical resistance is low, and the current concentrates therein. Therefore, unevenness of cell current occurs, and the whole of the cell reaction region cannot be utilized uniformly, which corresponds to a decreased effective cell reaction area, so that the cell performance such as output and efficiency decreases. Also, there arises a problem in that the cell is damaged by a thermal stress generated by a difference in temperature or the corrosion and degradation of material proceed in the high-temperature portion.
A fuel cell has been known in which in order to reduce the variations in temperature distribution in the cell, in a fuel cell having a tube shaped solid electrolyte, an oxidizer gas is supplied from not only a location near the bottom of tube but also a plurality of locations in the axial direction of the solid electrolyte to increase the reactivity (for example, refer to JP-A-3-238763 (claims)).
In the above-described prior art, the uniformity of temperature is achieved by decreasing an oxygen concentration gradient in an inside space of tube.
It is an object of the present invention to provide a fuel cell in which a high-temperature portion in a cell reaction region is cooled or a low-temperature portion therein is heated by a method different from that in the prior art, by which the uniformity of temperature distribution in the cell reaction region is achieved.